Diane Abbott has been criticised by climate change activists over her failure to sign a pledge calling for MPs’ pensions to be divested from fossil fuel companies.

The Divest Parliament campaign is demanding the Parliamentary Contributionary Pension Fund’s board of trustees “quantify, disclose and review” its investments from carbon-intensive industries and “commit to phasing out fossil fuel investments”.

The pledge of support has been signed by Meg Hillier, MP for Hackney South and Shoreditch, and 51 other MPs, most of them from the Labour Party, along with 22 former MPs and 325 parliamentary candidates for 2017.

Diane Abbott was contacted about the campaign by around 50 constituents, and wrote them a letter which expressed general support on the issue.

But the Hackney North and Stoke Newington MP has declined to sign the pledge of support, in a move which has left the campaign’s organisers “mystified”.

A spokesperson for Diane Abbott said “it’s usually protocol for members of the Shadow Cabinet not to sign petitions”, adding that this was more of a convention than a rule.

However, the pledge has been signed by at least three current Shadow Cabinet members, Barry Gardiner, Cat Smith and Rebecca Long-Bailey.

Ms Abbott’s spokesperson acknowledged this and suggested their signing might have been an “oversight”, saying she “would look into it”.

But she added: “If Diane Abbott doesn’t abide by the rules entirely, it’s a story. No other MP – well maybe Jeremy [Corbyn] – is subject to that kind of scrutiny.”

Sam Speaight, a spokesperson for Divest Parliament, told the Citizen: “Diane Abbott has not committed to divest the parliamentary pension fund of fossil fuels, and while we’ve received a letter of support from her office there’s no firm commitment from her on this issue right now.

Meg Hillier signed the petition in January. Photograph: Hackney Council

“She lives in a borough that’s populated with constituents who are concerned about issues like this, and the fact that she’s stopped short of pledging to sign the pledge has me and my team mystified.”

He added: “If we can’t rely on MPs who sit in seats like Hackney North and Stoke Newington to lead the people of this country toward a better future, what hope is there for the rest of the nation and indeed the world?”

Mr Speaight said he has contacted Diane Abbott’s office several times asking for support and received no response.

Diane Abbott’s letter said: “I agree that the Pension Fund’s refusal to provide answers on investment strategies or outline measures that are being taken to manage climate related financial risks is worrying.

“It raises serious ethical issues if MP’s pensions are being used to invest in companies that are complicit in contributing to climate change.

“Climate change is of paramount importance and it should be at the forefront of our policy agenda. It is critical that we are conscious of our environment for those present and for those who come after us. I have and will continue to advocate for better strategies to tackle climate change.

“It is imperative that the Parliamentary Contributory Pension Fund commits to acting in alignment with over 1,000 other organisations to reduce their exposure to the fossil fuel industry.”

The letter stops short of backing the campaign’s call for the trustees to disclose and review its investments and to phase them out over time.

Meg Hillier told the Citizen: “It is right that the Parliamentary Contributory Pension Fund should consider responsible investment practices, particularly when it comes to fossil fuels.” She signed the pledge in January 2017.

This article was amended at 6.20pm on Monday 11 December 2017. The original article stated that Sam Speaight, was managing director of Divest Parliament. This is not the case: he is in fact a spokesperson for the campaign group.